HU is a histone-like protein expressed in both mammalian and bacterial cells. In bacterial cells it seems to have a not well-understood role in DNA conformation in addition to gene regulation. Here, we examine the interaction of wild-type and mutant forms of HU with long, linear and plasmid DNA molecules using the atomic force microscope. Images are obtained of molecules and their complexes on freshly cleaved mica surfaces. These images are then analyzed qualitatively and quantitatively aiming at elucidating the mechanics of DNA compaction. Imaging is performed in air after mixing and incubating on freshly cleaved mica.